Considerable technical effort has gone into research in the preparation of cyanine dyes which function as spectral sensitizers and electron acceptors in direct positive photographic silver halide emulsions to give direct-positive images of good quality. Examples of known direct positive cyanine dyes are disclosed in Litzerman et al., U.S. Pat. No. 3,505,070, one of the cyanine dyes containing a desensitizing nucleus linked by a dimethine chain to a 1-alkyl-2-pyridyl substituted indole nucleus wherein the attachment to said methine chain is by the 3-carbon atom of the indole nucleus; and Chapman, U.S. Pat. No. 3,598,596, one of the cyanine dyes containing a desensitizing nucleus linked by a dimethine chain to a 1-aryl-2-arylindole nucleus wherein the attachment to said methine chain is by the 3-carbon atom of the indole nucleus. While the dyes of Litzerman et al., and Chapman are said by the inventors to exhibit good sensitometric performance, in general the dyes, when used in direct-positive photographic film, have a tendency to produce undesirable stain after imagewise exposure and development thereof.
Thus, there is a need to overcome the aforementioned stain disadvantage of previous direct-positive cyanine dyes and yet retain the advantages of improved sensitivity in direct-positive silver halide emulsions.